There is known a vehicular drive system including a differential mechanism operable to distribute an output of an engine to a first electric motor and its output shaft, and a second electric motor disposed between the output shaft of the differential mechanism and vehicle drive wheels. An example of such a vehicular drive system is a drive system for a hybrid vehicle disclosed in Patent Document 1. In this hybrid vehicle drive system, the differential mechanism is constituted by a planetary gear set, and a major portion of a drive force of the engine is mechanically transmitted to the drive wheels by the differential function of the differential mechanism, and the remaining portion of the drive force of the engine is electrically transmitted through an electric path between the first electric motor and the second electric motor, so that the drive system functions as an electrically controlled continuously variable transmission the speed ratio of which is continuously variable and which is controlled by a control apparatus to drive the vehicle while the engine is kept in its optimum operating condition, for thereby improving the fuel economy of the vehicle.
[Patent Document 1] JP-A-2003-301731
Generally, a continuously variable transmission is known as a device which permits improved fuel economy of the vehicle, while on the other hand a gear type transmission such as a step-variable automatic transmission is known as a device having a high degree of power transmitting efficiency. However, there is not available any power transmitting mechanism having the advantages of those two types of transmission. For example, the hybrid vehicle drive system disclosed in the above-identified Patent Document 1 includes the electric path for transmitting an electric energy from the first electric motor to the second electric motor, namely, a power transmitting path for transmitting a portion of the vehicle drive force as an electric energy, so that the first electric motor is required to be large-sized to meet a need for an increased output of the engine, and the second electric motor driven by the electric energy generated by the first electric motor is also required to be accordingly large-sized, whereby the overall size of the hybrid vehicle drive system tends to be large-sized. It is also noted that a portion of the output of the engine is once converted into an electric energy which is subsequently converted into a mechanical energy to be transmitted to the drive wheels, whereby the fuel economy of the vehicle may possibly be deteriorated under some running condition of the vehicle, for instance, during a high-speed running of the vehicle. Where the above-described differential mechanism is a transmission the speed ratio of which is electrically variable, for example, a continuously variable transmission so-called an “electric CVT”, the vehicular drive system suffers from a similar problem.
In the hybrid vehicle drive system of the above-identified Patent Document 1, it is well known to provide a transmission in a power transmitting path between the output member of the differential mechanism (electrically controlled continuously variable transmission) and the vehicle drive wheels, for the purpose of reducing the required capacity of the second electric motor when a high vehicle drive torque is required. In this vehicular drive system, the output of the drive power source is transmitted to the drive wheels through the two transmission mechanisms consisting of the electrically controlled continuously variable transmission and the transmission, and the overall speed ratio of the drive system is determined by the speed ratios of the two transmission mechanisms.
When a shifting action of the transmission takes place, the drive system as a whole may not be able to function as a continuously variable transmission, during the shifting action, unlike a drive system including only the electrically controlled continuously variable transmission. Therefore, the drive system may suffer from a shifting shock, and deterioration of the fuel economy due to a failure of the engine to operate in its optimum condition. A vehicular drive system provided with a transmission disposed in the power transmitting path between the differential mechanism and the drive wheels so as to solve the problem of the hybrid vehicle drive system described above also may suffer from the shifting shock and the deterioration of the fuel economy due to the failure of the engine to operate in its optimum condition.
The present invention was made in view of the background art described above. It is therefore an object of this invention to provide a control apparatus for a vehicular drive system including a differential mechanism operable to perform a differential function for distributing an output of an engine to a first electric motor and its output shaft, another electric motor disposed in a power transmitting path between the differential mechanism and a drive wheel of a vehicle, the control apparatus permitting reduction of the required size or an improvement of the fuel economy of the vehicular drive system, and reduction of the shifting shock of the vehicular drive system.